qtloader.js update

Updating qtloader.js

qtloader.js is currently written in old-style JS, with a feature set which has evolved over time. We want to modernize and and evaluate which features it should have.

See QTBUG-74295 for background. This bug tracks the current implementation project.

To be evaluated:

• Structure
  • Should the main API be a JS class (yes)
  • Should it be a JS module?
  • Should a QtLoader object control one wasm instance, or should it be possible to call load() multiple times and get multiple instances?
  • Should the main html file include the emscripten runtime, or should qtloader eval() it?

• Features
  • splash screen support
  • status change events: which type of events do we need, when are they changing?
  • ability to capture output
  • restarton-crash capability
  • support for multiple instances / external module compilation
  • platform capabilities test (webgl, wasm support)
  • support for setting environment variables / command line arguments
  • support for auto-test / batched tests use cases
• Misc
  • Change the “createQtAppInstance()” export name to include the app name. This allows using multiple applications on one page.
  • Simplify loading text “Downloading/Compiling” -> “Loading”